This invention relates in general to drafting instruments and in particular relates to compasses for drawing ellipses, polygons and similar geometric figures.
Ellipsographs have previously been provided for drawing variable ellipse figures. Examples are as disclosed in U.S. Pat. Nos. 3,719,996, 2,690,013, 2,498,798. However, the prior art instruments of this type have not been completely satisfactory for a number of reasons. The instruments are relatively unstable so that when pressure is applied as the figure is drawn the device tends to move out of adjustment. Furthermore, the prior art instruments cannot readily be adjusted because it is necessary to make repeated trial adjustments until the desired radii of the ellipse is attained. In the prior devices incorporating a multiplier arm, such as in U.S. Pat. No. 2,498,798, the angle of the multiplier arm changes as the angle of the cam is changed, with the unsatisfactory result that the ellipse being drawn is not symmetrical. Accordingly, the need has been recognized for a drafting instrument which will obviate the limitations and disadvantages of existing instruments.